In today's radio communications networks a number of different technologies are used, such as Long Term Evolution (LTE), LTE-Advanced, Wideband Code Division Multiple Access (WCDMA), Global System for Mobile communications/Enhanced Data rate for GSM Evolution (GSM/EDGE), Worldwide Interoperability for Microwave Access (WiMax), or Ultra Mobile Broadband (UMB), just to mention a few possible implementations.
In a typical cellular radio system, also known as radio communications network, wireless terminals, also known as mobile stations and/or user equipments (UEs), communicate via a radio access network (RAN) to one or more core networks. The RAN covers a geographical area which is divided into cell areas, with each cell area being served by a base station, e.g. a radio base station (RBS), which in some networks may also be called, for example, a “NodeB” for WCDMA or “eNodeB” for LTE. A cell is a geographical area where radio coverage is provided by the radio base station equipment at a base station site. The cell definition may also incorporate frequency bands used for transmissions, which means that two different cells may cover the same geographical area but using different frequency bands. Each cell is identified by an identity within the local radio area, which is broadcast in the cell. Another identity identifying the cell uniquely in the whole mobile network is also broadcasted in the cell. The base stations communicate over the air or radio interface operating on radio frequencies with the user equipments within range of the base stations. The user equipments transmit data over the radio interface to the radio base stations in uplink (UL) transmissions and the radio base stations transmit data over an air or radio interface to the user equipments in downlink (DL) transmissions.
In some versions of the RAN, several base stations are typically connected, e.g. by landlines or microwave, to a controller node, such as a Radio Network Controller (RNC) or a Base Station Controller (BSC), which supervises and coordinates various activities of the plural base stations connected thereto. The RNCs are typically connected to one or more core networks.
Multi-Carrier (MC) High Speed Downlink Packet Access (HSDPA) transmission was standardized in 3rd Generation Partnership Project (3GPP) Releases (Rel)-8/9/10/11. This allows a wireless User Equipment (UE) to simultaneously receive data transmissions from multiple cells that belong to the same sector, same sector meaning same geographical area. This allows the use of one High Speed-Dedicated Physical Control Channel (HS-DPCCH) carrying the Hybrid Automatic Repeat Request Acknowledgement (HARQ-ACK) and Pre-Coding Information/Channel Quality Indicator (PCI/CQI) feedback for all cells, without compromising the HARQ-ARQ time budget for the base station or user equipment.
At a RAN#53 plenary of 3GPP, a work item on Multi-Flow (MF) HSDPA was initiated, as detailed in document RP-111375, “HSDPA Multiflow Data Transmissions,” the disclosure of which is incorporated by reference herein in its entirety. MF-HSDPA transmissions allow a user equipment to receive data from different, potentially uncoordinated, cells. Further, an agreement was made that MF-HSDPA transmission will be supported in combination with 2×2 Multiple Input Multiple Output (MIMO).
FIG. 1 depicts a situation where a user equipment is in soft handover with two or more base stations and the uplink of the user equipment is power controlled by the strongest base station. In FIG. 1, the two base stations are a macro base station and a low-power base station, such as one implemented in a traffic hotspot. For MF-HSDPA scenarios, where both base stations schedule downlink transmissions to the user equipment, it is important that all base stations from which downlink transmission occurs can receive the feedback channel, e.g. HS-DPCCH, carrying the HARQ-ACK and PCI/CQI information related to the downlink cells.
In the 3GPP specifications, as well as in practice, there exist several classes of base stations, e.g. macro, micro, etc., which may be characterized by their transmit power, as well as other parameters. One scenario where such different base stations co-exist is when a macro layer, i.e. coverage from macro base station, is complemented by a small-power base station in a traffic hotspot. In order to benefit from MF-HSDPA transmissions, the received signal power from all participating base stations must be on par with each other. In FIG. 1, a user equipment is in a soft handover region of a macro and a small-power base station. An optimal position for UL handover is marked and an optimal position for DL handover is also marked. For this type of deployment, the strongest downlink base station is not necessarily the best base station from an uplink perspective. Furthermore, if a serving cell change is based on downlink Ec/lo or Received Signal Code Power (RSCP), then there can be a considerable uplink imbalance between the two base stations.
Table 1 depicts the quantized gain factors supported up until 3GPP Rel-10:
TABLE 1Summary of the list of supported gainfactors for HS-DPCCH physical channelSignaled valuesDifference in Tx powerfor ΔACK, ΔNACK andQuantized amplitudebetween adjacentΔCQIratios Ahs = βhs/βcsignaled Δ values1048/152.03938/152.05830/151.94724/152.03619/152.05515/151.94412/152.53 9/151.022 8/152.51 6/151.580 5/15—
It should be noted that the network can only signal values between 0 and 8 via the Radio Resource Control (RRC) signaling, i.e. from the RNC. The offset corresponding to value 9 or 10 is used by the user equipment for certain MC-HSPA and MIMO configurations. The suitable power offsets ΔACK, ΔNACK and ΔCQI for MF-HSDPA will be implementation-specific and the link imbalance that could be handled by the existing signaling, as a function of the signaled Δ-values, is shown in FIG. 2. Δ-values are defined along a horizontal axis and maximum asymmetry in dB is defined along a vertical axis. From this FIG. 2, it is shown that if the same power is used for DPCCH and HS-DPCCH, i.e. Δ=5, a total link imbalance, accounting both for the difference in fast fading and the differences in path loss, of 6 dB could be handled by adapting the power offsets used for the HS-DPCCH transmissions. However, it is questionable whether the existing power offsets are sufficient to ensure an acceptable HS-DPCCH quality when the user equipment is in a Soft Hand-Over (SHO) region between a macro and a small-power base station. Furthermore, in current specifications, the reconfiguration would need to be based on RRC reconfiguration controlled by the RNC. This means that the serving base station would have to: Identify that the HS-DPCCH quality is inferior; Signal to the RNC that the HS-DPCCH quality is inferior; and the RNC would need to re-configure the power offsets used by the user equipment.
These steps are associated with significant delays, e.g. ˜several 100s of ms, and the RRC reconfiguration would furthermore be sent to a user equipment in poor coverage, at least for the HS-DPCCH, since this is what triggered the reconfiguration in the first place. Hence, the transmissions of these RRC reconfigurations would be associated with low quality.